1. Field of the Invention
The invention concerns a method to adjust and a method to operate a hearing aid device wearable on the body of a test person, having a microphone system disposed outside of the auditory canals when the test person wears the hearing aid device.
The invention also concerns a hearing aid device wearable on the body of a test person, with a signal processing unit and a microphone system disposed outside when the auditory canals of the test subject wears the hearing aid device.
2. Description of the Prior Art
If a person is located in a natural sound field, sounds reach the eardrums of both ears from different directions with different levels, durations and frequency weighting. The capability of the person to localize (i.e. identify the originating location of) different signal sources in the sound field is based substantially on the existence in the horizontal plane of interaural level and duration differences. For the most part, head shadowing effects and the direction-dependent transmission characteristic of the external ears are responsible for the dependent level and duration differences of the sound incidence direction. The elevation perception (localization ability in the vertical direction) is based almost exclusively on the elevation-dependent spectral modification of the sound signal through the external ears.
For a person wearing a device having microphones disposed outside of the auditory canals, for example behind-the-ear (BTE) devices, the spectral modification via the external ears does not occur, so that important directional and elevation information is lost. The results are the known localization problems (for example forward/behind confusion) of hearing impaired persons wearing a BTE device. The interference of the spatial acoustic orientation (and with it the sound quality) connected with this often contributes to dissatisfaction with the device.
To solve this problem, in-the-ear (ITE) hearing aid devices can be used, however, with these at best small and medium hearing losses are compensated. Moreover, as a rule they are more expensive than BTE hearing aid devices and are more subject to interfering feedbacks.
In order to determine the acoustic pressure that an arbitrary signal source produces preceeding the eardrum of a person; it is sufficient to know the pulse response between the source and the eardrum. This is called HRIR (Head Related Impulse Response). Its Fourier transformations are called HRTF (Head Related Transfer Function). The HRTF comprises all physical parameters for localization of a signal source. If the HRTFs are known for the left and the right ear, binaural signals can also be synthesized from an acoustic source.
In echo-free surroundings, the HRTF is a function of four variables: the three spatial coordinates (with regard to the head) and the frequency. To determine the HRTFs, for the most part measurements are implemented on a synthetic head, for example the KEMAR (Knowles Electronics Mannequin for Acoustical Research). An overview of the determination of HRTFs is, for example, known from Yang, Wonyoung, “Overview of the Head-Related Transfer Functions (HRTFs)”, ACS 498B Audio Engineering, The Pennsylvania State University, July 2001.
It is known from the field of synthetic head technology that the direction-dependent transfer functions of the head and the external ear can be relatively precisely simulated by multi-microphone arrangements in a free field with suitable subsequently circuited filters (for example Podlaszewski, Mellert: “Lokalizationsversuche für virtuelle Realität mit einer 6-Microfonanordnung”, DAGA 2001). The filters are thereby designed with specific optimization methods such that the sum of the filtered microphone signals (typically 3 per side) for any spatial direction corresponds with a known error tolerance of the sound signal that was measured in the ear canal in the synthetic head in the same situation.